U.S. Pat. No. 4,944,511 to Francis is directed to a resistance exercise machine in which the resistive force is provided by stacked reels containing springs that are arranged to resist turning of the reels. This type of resistance system can function adequately in many applications. However, the springs can lose their effectiveness after extended use. Furthermore, springs provide increasing resistance as they are progressively deformed. Consequently, the last parts of an exercise movement are characterized by more resistance than the first parts. This inconsistent force over the full range of movement can be a significant disadvantage.
U.S. Pat. Nos. 6,126,580 and 6,440,044 to Francis et al. address the problem of inconsistent resistance in two different ways. First, resistance packs having deformable spokes are connected in a series arrangement that allows the actuator cord to be displaced a lengthy distance without a great variation in the resistance force. Second, a spiral pulley is provided to increase the moment arm with increasing displacement of the actuator cord in order to counteract the increasing resistance force.
Although this type of approach is generally satisfactory, it is disadvantageous because compensation can be made for the inconsistent force only within a relatively limited resistance range. Also, a somewhat complicated preload mechanism is required in order to adjust the resistance force. The spiral pulley that is used also adds to the cost and complexity of the resistance mechanism and to the amount of space that it requires.